Leetcode:Binary Tree Zigzag Level Order Traversal

Description:

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

[
[3],
[20,9],
[15,7]
]

分析： 题目要给出二叉树的zigzag Z型遍历，其本质应该是二叉树的宽搜，然后根据层数来确定每一层是否翻转。

这主要点就在于宽搜时需要知道当前在第几层，所以需要两个queue来做。 这个是一个常用技巧

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
vector<vector<int> >result;
if(root==NULL) return result;

queue<TreeNode* > q,qadd;
q.push(root);
vector<int> level;
int nowlevel = 1;
while(!q.empty())
{
TreeNode *one = q.front();
level.push_back(one->val);
if(one->left!=NULL) qadd.push(one->left);
if(one->right!=NULL) qadd.push(one->right);

q.pop();
if(q.empty())
{
if(nowlevel%2==0) reverse(level.begin(),level.end());
result.push_back(level);
level.clear();
nowlevel++;
swap(q,qadd);
}
}
return result;
}
};